We have previously found that the ventral mesoderm explant, which alone was not able to differentiate into erythrocytes, can be stimulated for erythropoiesis in combination with animal pole cells (st. 10). To characterize the nature of this stimulation, different sizes, portions, and stages of animal pole tissue were combined with the ventral mesoderm explants. We demonstrated that the animal pole tissue at earlier stage (st.7) failed to stimulate erythroid differentiation in the ventral mesoderm. No morphological difference was observed in the explants with st.7 and st. 10 animal pole tissues, but both mRNA and protein levels of globin expression were not stimulated in those with st.7 animal pole tissues. However, total RNA or poly(A)+RNA from the St. 10 animal pole tissue can restore the stimulation in the explants with st.7 animal pole tissue, if previously injected. These experiments strongly suggest that there exist substances expressed after st.7 in the animal pole region, which can stimulate the erythropoiesis in the ventral mesoderm. Furthermore, we have tested the effect of RNAs of stem cell factor (SCF), bone morphogenetic proteins (BMP) in the above system, and shown that BMP- 2 and -4, as well as SCF RNAs, stimulate the erythropoiesis. In studies of BMPs, we also cloned two cDNAs encoding Xenopus homolog of BMP-1 from embryonic library. Xenopus BMP-1 protein shares 82% identity with the human counterpart, and three transcripts (2.9, 5.2, and 6.6 Kb) were detected in early gastrula (st. 10) an in hatched tadpole (st.40) but not in morula (st.7) and late gastrula (st. 12), suggesting physiological role of this protein in the early development. Furthermore, we have isolated 3 clone which showed activities to induce erythrocytic differentiation in stage 10 ventral mesoderm. We are in the process of characterizing these clones. The discovery of these novel genes involved in the erythropoiesis will help us to elucidate the mechanism in the regulation of erythrocytic differentiation.